We are very supportive of alternative ways to use the vast timber and fibre resources of British Columbia. Engineered wood products employ a lot of people here and have produced some amazing structural materials to replace traditional posts and beams.
For exterior wall sheathing there are two engineered materials made and commonly used in this province: plywood, and OSB, oriented strand board. Both replaced traditional shiplap dimensional lumber a long time ago, and for good reason, because shiplap has serious structural limitations.
Both are widely used. From our experience, only one of them should be.
How plywood behaves when it gets wet
Plywood is created by laminating veneers of softwood, generally spruce or fir, with layers of waterproof adhesive in a laminar form. Spruce is a reasonably durable softwood. Fir is extremely durable. The veneers are generally fully coated between each ply with waterproof glue.
If moisture reaches plywood, it generally does not cause decay in a hurry. When it does, the decay is often confined to the outermost surface without structurally compromising the sheet.
How OSB behaves when it gets wet
OSB is created by gluing together an amalgamation of strands of various softwood species. Not generally fir, but spruce, pine and poplar. Pine and poplar have very low durability once wet.
The strands are also oriented in various directions, which lets decay, once it is occurring, spread much further into the sheet. There is no solid laminar layer of waterproof glue to stop it. OSB is coated with waxes to make the exterior more water repellent, but that seems to do little once the material is cut or penetrated with fasteners and then gets wet.
What we have actually found
“We have taken moisture damaged walls apart where both OSB and plywood have been used in the same area for sheathing and found intact but water stained plywood right next to completely decayed OSB.”
Same wall. Same exposure. Same water. Two completely different outcomes.
Where we land. We believe that, when it comes to exterior sheathing, plywood is the only way to go.
It costs more per sheet. It is a rounding error against the cost of opening a wall back up in fifteen years.
